Low-noise spectroscopic ellipsometer

ABSTRACT

A spectroscopic ellipsometer comprising a light source ( 1 ) emitting a light beam, a polarizer ( 2 ) placed on the path of the light beam emitted by the light source, a sample support ( 9 ) receiving the light beam output from the polarizer, a polarization analyzer ( 3 ) for passing the beam reflected by the sample to be analyzed, a detection assembly which receives the beam from the analyzer and which comprises a monochromator ( 5 ) and a photodetector ( 4 ), and signal processor means ( 6 ) for processing the signal output from said detection assembly, and including counting electronics ( 13 ). Cooling means ( 12 ) keep the detection assembly at a temperature below ambient temperature, thereby minimizing detector noise so as to remain permanently under minimum photon noise conditions. It is shown that the optimum condition for ellipsometric measurement is obtained by minimizing all of the sources of noise (lamps, detection, ambient).

GENERAL FIELD AND STATE OF THE ART

[0001] Background

[0002] The principles of a known state of the art ellipsometer are shownin FIG. 1.

[0003] Such an ellipsometer conventionally comprises a light source 1, apolarizer 2, an analyzer 3, and a detector 4 associated with amonochromator 5.

[0004] Those various elements are placed in such a manner that lightoutput by the source 1 passes thorough the polarizer 2 before reaching asample E to be analyzed, and then after being reflected on the sample E,it passes through the analyzer 3 prior to reaching the detector 4 afterpassing through the monochromator 5 which is generally aphotomultiplier.

[0005] One of the two elements constituted by the polarizer 2 and theanalyzer 3 is a rotary element.

[0006] The output from the detector 4 is connected to processor means 6which perform Fourier analysis on the modulated signal as measured bythe detector 4 in order to determine information relating to the surfacestate of the sample E.

[0007] It is recalled that when light is reflected on a sample E, itspolarization is modified and that an ellipsometer setup makes itpossible to measure firstly the phase difference Δ and secondly theratio tan (ψ) between the parallel and perpendicular polarizationcomponents of the beam as reflected on the sample.

[0008] By means of the monochromator 5, it is possible to performmeasurements at different wavelengths, thereby characterizing theoptical properties of the material.

[0009] For a general presentation of spectroscopic ellipsometrictechniques, reference can advantageously be made to U.S. Pat. No.5,329,357 (Bernoux et al.) which relates specifically to the advantageof adding optical fibers to the setup.

[0010] The visible spectroscopic ellipsometers available on the marketgenerally operate in a spectral range of 1 micrometer (μm) to 230nanometers (nm), using a xenon arc source (selected for high radiantflux density or “irradiance”).

[0011] Nevertheless, ellipsometers have been proposed that are capableof operating over a broader spectral range than the above-mentionedellipsometers and that include an additional source, such as a deuterium(D₂) source that provides less of a point source, emitting in the range130 nm to 700 nm at a power of 30 watts (W) to a few hundreds of wattsor more.

[0012] The detectors that are used are generally detectors of the Si orGe photodiode type or photomultipliers (generally multi-alkaliphotomultipliers), operating at ambient temperature.

[0013] They use very high quality optical systems, possessingpolarization extinction coefficients of about 10⁻⁵°, and very hightransparency, even in the ultraviolet.

[0014] This makes it possible in the above-specified spectral range todetermine the ψ and Δ coefficients with precision equal to or less than{fraction (1/1000)}^(th) of a degree (°).

[0015] Furthermore, the processor means of most ellipsometers implementa simplified photon counting method, which method is known as the“Hadamart method”. That method consists in counting photons with asignal that is amplitude sampled over a very limited number of channels:eight counters or channels, for each period of rotation of the rotaryelement of the ellipsometer (a configuration with a rotary polarizer oranalyzer (modulated polarization) and/or a rotating plate (phasemodulation)).

[0016] Drawbacks of State of the Art Ellipsometers

[0017] Ellipsometers of the type described above present severallimitations.

[0018] A first limitation is associated directly with fluctuations inthe source, i.e. with its lack of stability, with this constraint beingknown as shot noise limitation (SNL).

[0019] Another limitation is associated with noise coming from ambientlight and also referred to as “leakage noise”, which can in theory beeliminated by isolating the entire ellipsometer (and not only thephotomultiplier) completely from ambient light, but which neverthelessremains a difficulty encountered by many ellipsometer manufacturers.

[0020] Another limitation lies in the dark current or intrinsic noiseassociated with the photomultiplier and its internal amplificationsystem. This noise is commonly referred to as detector noise limitation(DNL). It should be observed that all of the frequencies correspondingto the bandwidth of the photomultiplier are generally present therein.

[0021] Thus, the Hadamart sums (as determined over quarter periods ofthe modulated signal) are calculated by taking account of a previouslymeasured offset which corresponds to the leakage noise and to the DNL.

[0022] Nevertheless, although conventional ellipsometers correspond inpractice to synchronous detection (in-phase frequency filtering of thesignal modulation), the Hadamart method becomes difficult when theamplitude of the modulation is low.

[0023] For a signal modulated at ω, the amplitude of the spectrumcomponent at 2ω in the signal is of the same order of magnitude as theamplitude of the noise (with this being true more particularly in theultraviolet where counts of only 100 to 1000 counts per second (cps) aremeasured).

[0024] The signal components are thus “buried” in the noise level whichitself corresponds to a superposition of the spectrum density of thesource noise, shot noise when using a xenon arc, ambient light, andnoise from the detector and its associated electronics.

[0025] Furthermore, with conventional ellipsometers, when it is desiredto work at wavelengths shorter than 200 nm, the observed signal/noiseratio is unfavorable.

[0026] The only known way of eliminating the effects of the varioussources of noise is to increase acquisition times.

[0027] Unfortunately, measurement is then subject to systematic error,in particular concerning wavelengths shorter than 200 nm. This meansthat equipment must be pre-calibrated in use.

[0028] Furthermore, it should also be observed that another problemencountered with ellipsometers that use additional sources to enlargetheir operating range is the problem of their cost and of the power thatmust be supplied to them.

[0029] Under such conditions, it is practically impossible to envisage asystem that is sufficiently compact for in situ measurement (integratedmetrology) even in a photon-counting system as described above. Theimpossibility of having a measurement head internal to the metrologicalcasing leads to a limitation due to the windows of the casing givingrise to birefringent effects that need to be corrected.

SUMMARY OF THE INVENTION

[0030] An object of the invention is to mitigate those drawbacks.

[0031] In particular, the invention provides an ellipsometer structurein which noise is minimized.

[0032] Techniques are known, in particular from the abstract of Japanesepatent application No. 0907995, that consist in cooling photomultipliersin applications that are very different from ellipsometer applications.

[0033] Those cooling techniques are not intended in any way to reducenoise. They serve to lower detection limits as much as possible.

[0034] The invention proposes a spectroscopic ellipsometer comprising alight source emitting a light beam, a polarizer placed on the path ofthe light beam emitted by the light source, a sample support receivingthe light beam output from the polarizer, a polarization analyzer forpassing the beam reflected by the sample to be analyzed, a detectionassembly which receives the beam from the analyzer and which comprises amonochromator and a photodetector, and signal processor means forprocessing the signal output from said detection assembly, and includingcounting electronics.

[0035] This ellipsometer presents the characteristic of comprisingcooling means for keeping the detection assembly at a temperature lowerthan ambient temperature, in particular at a temperature of about −15°C., or lower.

[0036] Also advantageously, its source is a deuterium lamp preferablyhaving a power of about 30 watts.

[0037] Other low noise sources can be envisaged, and in particularplasma lamp and halogen lamp sources.

[0038] Also advantageously, the counting electronics is suitable forperforming amplitude sampling over a number of channels lying in therange 8 (Hadamart equivalent) up to 1024 (filtered Fourier), andparticularly preferably about 1000 or more, in particular a number ofchannels lying in the range 1024 to 8192 (depending on the type ofencoder).

[0039] The processing means implement Fourier analysis on the signalssampled in this way.

[0040] Thus, the proposed ellipsometer enables noise to be minimized (toimprove its precision): i) with total protection from ambient light; ii)no polluting environment (mechanical vibration and/or sources ofelectromagnetic noise); and iii) a detector operating by countingphotons in a minimum intrinsic noise level which is obtained in thiscase by cooling (12) to keep the detection assembly at a temperaturelower than ambient temperature. By providing better performance in termsof signal detection, it makes it possible to use optical fibers all theway to 160 nm. This makes it possible to operate in compact manner inthe context of integrated metrology associated with current developmentof cluster tools in the field of thin layer technology. The systembecomes fully integrated in the in situ casing since it makes itpossible to use films.

BRIEF DESCRIPTION OF THE FIGURES

[0041] Other characteristics and advantages of the invention appearfurther from the following description which is purely illustrative andnon-limiting and should be read with reference to the accompanyingdrawings, in which:

[0042]FIG. 1, described above, is a diagram illustrating the principlesof a spectroscopic ellipsometer known in the state of the art;

[0043]FIG. 2 is a block diagram of an ellipsometer constituting anembodiment of the invention;

[0044]FIGS. 3a and 3 b are graphs on which measurements of theparameters ψ and Δ are plotted as a function of wavelength for anellipsometer as shown in FIG. 2 and for a standard ellipsometer; and

[0045]FIG. 4 is a graph showing direct trace measurements obtained by anellipsometer as shown in FIG. 2 and by a standard ellipsometer, themeasurements being plotted as a function of wavelength.

DESCRIPTION OF POSSIBLE EMBODIMENTS OF THE INVENTION

[0046] Background

[0047] A spectroscopic ellipsometer constituting a possible embodimentof the invention presents the following characteristics.

[0048] 1) Its source 1 is a low noise source, i.e. a source whosefrequency dispersion is much less than that of a xenon arc lamp.

[0049] Such a source is advantageously a deuterium D₂ lamp.

[0050] Deuterium D₂ lamps are lamps having particularly low noise. Theyare much more stable than xenon arc lamps (stability in a ratio of 20),and they also provide much better stability than other types of lamp inthe ultraviolet and in a portion of the visible.

[0051] Other types of lamps could be envisaged. In particular halogenlamps (towards the infrared) or plasma discharge lamps (visible and UV),or plasma sources are particularly suitable, even when they haveemission spectrum lines (non-continuous spectrum).

[0052] 2) Its photomultiplier 5 is placed together with magneticprotection in a low temperature environment, thereby reducing andstabilizing its dark current.

[0053] For this purpose, a Peltier effect cooling system is used (−15°C.).

[0054] By reducing the dark current of the photomultiplier, noise isreduced by at one least decade (from 200 cps to 10 cps, for example).

[0055] Thus, the above-mentioned effects of offset instability areeliminated.

[0056] Temperatures that are even lower further improve the performanceof the photomultiplier to a few counts per second.

[0057] In a spectral range such as the ultraviolet, when using a D₂lamp, source emission noise is low. DNL is then of the same order ofmagnitude as the SNL limit. This amounts to saying that the number ofdark current counts N_(d) continues to be negligible, and the signal isdegraded solely by a low level of residual source noise N_(ph) (numberof photons due to source emission). Because of the intrinsic darkcurrent of the photomultiplier is reduced, precise ellipsometicmeasurement is obtained, which is not possible when N_(ph)≦N_(d).

[0058] This reduction in dark current makes it possible in particular toperform measurements at 187 nm, with a low power lamp and in anenvironment that is not restricted (no vacuum, no purging with an inertgas, no dry atmosphere). The photomultiplier operates in a photoncounting mode and ideally it is linear (no non-linearity associated withavalanche overlap effects (saturation in analog mode)).

[0059] This limit of 160 nm can also be crossed by using inert gasconditions (nitrogen) with a PMT (R7639 from Hamamatsu) and acorresponding monochromator. In the extreme ultraviolet (120 nm to 160nm), it is advantageous to cool the base of the photomultiplier 5, or toperform N₂ sweeping in the photomultiplier enclosure cooled by a coolerelement of the Peltier heat exchanger type, thus avoiding the need for aMgF₂ window.

[0060] 3) Furthermore, the proposed spectroscopic ellipsometer countsover a large number of channels: up to more than 1024 channels, whichcombined with Fourier analysis oversamples the signal and provideseffective filtering of high frequency noise components, i.e. ofintrinsic noise from the sources and the detector and the electronics.

[0061] The time required for counting is reduced.

[0062] This counting technique turns out to be much better than thatwhich is possible using the Hadamart method. It can be implemented verysimply by using commercially available electronics. The oversampling itprovides contributes to attenuating noise at all of the high frequenciesassociated with lamp noise.

[0063] Detailed Example of One Possible Embodiment

[0064] An example of an ellipsometer constituting a possible embodimentis described below in detailed manner with reference to FIG. 2.

[0065] This ellipsometer is of the type having a rotary polarizer.

[0066] It operates in the range 180 nm to 750 nm. It can be used in avacuum or in a controlled atmosphere so as to extend its operating rangeto the range 130 nm to 720 nm.

[0067] The source 1 is a deuterium lamp (D₂) having power of 30 W and apoint source diameter of 0.5 millimeters (mm) (Oriel 63163 lamp orHamamatsu L7295 or L7296 lamp).

[0068] The light beam is transferred from the source 1 by means of a 600μm single strand fused silica fiber 7 to the rotary polarizer 2. One ofthe functions of the fiber is specifically to eliminate the residualbirefringence of the source 1.

[0069] Coupling with the rotary polarizer 2 is performed via aconverging element 8 of fused silica, selected for its low residualbirefringence and its transparency in the ultraviolet.

[0070] It can also be implemented using an assembly comprising a concavemirror and a plane mirror, both having MgF₂ surface treatment.

[0071] The sample E for analysis is placed at the outlet from thepolarizer 2 on a support 9 whose orientation can be adjusted.

[0072] The beam reflected by the sample is applied to the analyzer 3.

[0073] Both the polarizer 2 and the analyzer 3 are made of MgF₂ (forexample they are constituted by analyzers and polarizers fromFichou/Optique which certifies 2.50 of deviation at 250 nm and apassband to 10 electron volts (eV)). This choice of material makes itpossible to obtain greater transparency in the ultraviolet.

[0074] The polarizer 2 is rotated at a frequency of about 10 Hz (withthe criteria for selection being associated with the environment, mainsfrequency or vibration frequency) and is controlled by a mechanicalassembly of the stepper type (microstep).

[0075] After being reflected on the sample and passing through theanalyzer 3, the beam is refocused by a set of mirrors 11 and is appliedto the inlet of the monochromator 5 which is a dual monochromator havingan Oriel 77250 type {fraction (1/8)} M grating blazed at 250 nm with1200 lines (180 nm to 500 nm in first order and an intermediate 0.6 mmslot; its resolution at 500 nm is 4 nm). Gratings blazed at 200 nm butwith 600 lines per millimeter (mm) can be used.

[0076] While performing a measurement, the system automaticallyincorporates two filters in succession so as to eliminate higherdiffraction orders from the gratings of the monochromator. Control isperformed by means of an Oriel filter passer and an Ni DAQ (TTL)interface from National Instrument.

[0077] The output from the monochromator 5 is applied to the detector 4which serves to count photons. The detector 4 is of the tube type and itis sold by Hamatsu under the reference R2949 or R7639.

[0078] The detector 4 is placed in a cooler 12 of the C-659S type whichmaintains it at a temperature of −15° C.

[0079] The counting electronics includes a discriminator 13 connected tothe detector 4. The discriminator is of the type sold by Hamatsu underthe reference C 3866 and it has a linear dynamic range of 10⁷.

[0080] The detector 4 and the discriminator 13 are selected for theirlow dark current characteristics (159 cps at 25° C. and dropping to lessthan 10 cps when cooled for the R2949 and to less than 1 cps for theR7639 (which has quantum efficiency of 44% at 160 nm)). This can beimplemented using water cooling or “cryogenic” nitrogen flow coolingwith external cooling being provided by Peltier cooling elements and anexternal heat exchanger. The detector and the discriminator are alsoselected for their sensitivity in the blue of 8.3 μA/lm with gain of10⁷. The photon counting electronics is linear for 10⁷ photons.

[0081] The TTL output from the discriminator 13 is analyzed by means ofa multiscale count card 14 (MCS II Nuclear Instrument or FMS CanberraElectronics card CM 7882) capable of analyzing 8192 count channels, withtwo simultaneous inputs and a sampling time of 2 μs.

[0082] The card 14 is controlled by a computer 15 operating in a WindowsNT server environment with object C++ programming coupled withcommercial active X components, in this case the Works++ components fromNational Instruments.

EXAMPLES OF RESULTS

[0083]FIGS. 3a and 3 b show measurements of ψ and Δ obtained over awavelength range of 1 nanometer (around 250 nm) respectively when usinga standard xenon lamp ellipsometer, photomultiplier at ambienttemperature and Hadamart detection, and when using an ellipsometer asdescribed above.

[0084] It can be seen that measurements are much more widely dispersedwith the standard ellipsometer than with an ellipsometer of the typedescribed above.

[0085] Direct traces obtained with each of the two ellipsometers havealso been compared.

[0086] This is shown in FIG. 4.

[0087] The improvement is also clearly visible.

[0088] It is recalled that for a direct trace, it is necessary that tanψ=Cos Δ=1.

[0089] This turns out to be exactly true (ignoring optical alignment)for the above-described low noise spectroscopic ellipsometer, whereaswith a standard ellipsometer, the difference is much larger for Cos Δwhich means that it is then important to normalize α and β.

[0090] One application consists in extending an in situ setup. Therotary polarizer system and the analyzer are MgF₂ lumps mounted in avacuum on stepper micromotors (vacuum technology) having a hollow shaft(in which the MgF₂ lump is inserted) and the optical encoder which canthus be positioned even inside a casing or a cooling and measurementchamber of a cluster tool type reactor. The source and analysis inputsare then compact blocks. This makes it possible to implement two heads(analyzer and polarizer being equivalent). An estimate of the physicalsize that can be achieved corresponds to a cylinder having a diameter ofabout 40 mm and a length of 60 mm to 70 mm. Windows which are sources ofbirefringence and of absorption are thus eliminated since only theoptical fibers are connected to the casing of the reactor. It has beenshown that such a system can operate in situ for photons havingwavelengths in the spectrum range 160 nm to 170 nm.

1/ A spectroscopic ellipsometer comprising a light source (1) emitting alight beam, a polarizer (2) placed on the path of the light beam emittedby the light source, a sample support (9) receiving the light beamoutput from the polarizer, a polarization analyzer (3) for passing thebeam reflected by the sample to be analyzed, a detection assembly whichreceives the beam from the analyzer and which comprises a monochromator(5) and a photodetector (4), and signal processor means (6) forprocessing the signal output from said detection assembly, and includingcounting electronics (13), the ellipsometer being characterized in thatit further comprises cooling means (12) for keeping the detectionassembly at a temperature lower than ambient temperature. 2/ Anellipsometer according to claim 1, characterized in that said coolingmeans (12) are suitable for keeping the detection assembly at atemperature of about −15° C. or lower. 3/ An ellipsometer according toany preceding claim, characterized in that the source (1) is constitutedby a deuterium lamp. 4/ An ellipsometer according to claim 3,characterized in that the lamp has power of about 30 watts. 5/ Anellipsometer according to claim 1 or claim 2, characterized in that thesource is constituted by a cold plasma lamp. 6/ An ellipsometeraccording to claim 1 or claim 2, characterized in that the source isconstituted by a halogen lamp. 7/ An ellipsometer according to anypreceding claim, characterized in that the counting electronics (13) issuitable for performing amplitude sampling over a number of channelsequal to about 1000 or more. 8/ An ellipsometer according to claim 6,characterized in that the counting electronics (13) is suitable forimplementing amplitude sampling over a number of channels lying in therange 1024 to
 8192. 9/ An ellipsometer according to claim 7 or claim 8,characterized in that the processor means (6) apply Fourier analysis tothe signals output by the counting electronics.